1. Field of the Invention
The present invention relates to a method for making porous metals, and more particularly, to a method for making porous metals containing consecutive pores of 60-95%, which can be used as a filter, a sound-absorbing plate and a line material for a heat exchanger, In a simple process using metal powder.
2. Description of Prior Art
Porous metals are materials in which consecutive pores have been formed inside substances such as stainless steel, copper alloy, and aluminum alloy. The porous metals are used as a plastic molding filter, an air filter and an oil purification filter, by adjusting the size of the consecutive pores. Also, the porous metals are being noted as a sound-absorbing material by use or an oscillation attenuation phenomenon of sound in the pores. In particular, since it proved that asbestos or glass fibers used for sound-absorbing materials that have been used in abundance in the past are strong carcinogenic substances, they have been replaced with porous metals. In addition to a sound-absorbing material for use in a filter, porous metals can be used as line materials for a heat exchanger or radiator. This is because porous metals can suffice properties of line materials requiring a high thermal conductivity as well as a wide specific surface area.
As described above, in order to use porous metals in the above filters and sound-absorbing materials, pores in the substances should be consecutively connected, a porous ratio should be 60% or more, and a proper mechanical strength should be maintained.
Technologies that have beer developed so far follow with respect to porous metal manufacturing method sufficing the above requirement:
1) Method for thermally hardening metal powder at low density
2) Method for foaming fusion metal with a foaming agent;
3) Method for pressingly casting fusion metals in a perform of sodium chloride; and
4) Method for injecting gypsum slurry in a polyurethane form, thermally decomposing polyurethane, fabricating gypsum mold, and then vacuum absorbing fusion metals.
Among the above methods, the first method has a simple process and an excellent continuity of pores, which have been widely used. However, it is difficult to expect to have a porous ratio of 60% or more and is difficult to be adapted in aluminum alloys.
The second method has a simple process but should use an expensive foaming agent.
In the third method, sodium chloride is molded to have proper pores in advance, fusion metal is pressingly penetrated into the pores, and then sodium chloride is erupted in water to be removed. However, since most metals are extremely low in appetence with sodium chloride, fusion metal Is not nearly penetrated into pores of 1 mm or less even under the considerable pressure using a high-pressure caster. Also, penetration of fusion metal has a considerable difference in the surface and inside the sodium chloride. Thus, the third method has a limitation that it is used for making a rough porous metal having pores of several millimeters in size.
The last method is an application of a precise casting method, and has unique characteristics having a porous ratio of 90% or more and a three-dimensional reticulation, which is appropriate for a filter substance. However, the last method has a complicated process to thereby cause a high cost, and an internal metal of a reticular structure has a small specific surface area. Thus, it is difficult to use the last method in a sound-absorbing material and a line material for a heat exchanger that require a large specific surface area.
As described above, the existing methods for making porous metals have drawbacks such as a low porous ratio, a high cost due to a complicated process, or a small specific surface area.
To solve the prior art problems, it is an object of the present invention to provide a method for making porous metals having high productivity due to a simple process, a high porous ratio and a high specific surface area.
To accomplish the above object of the present invention, according to the present invention, there is provided a method for making porous metal comprising the steps of: heating a mixture of a powder-shaped salt and metal powder at a temperature lower than a melting temperature of the salt and higher than a melting temperature of metal powder, to thereby melt the metal powder; pressing and molding the mixture so that the fusion metal is filled into the powder-shaped salt; and erupting the salt from the plastic body to thereby obtain porous metal.
The kind of the salt that is used in the present invention is selected taking a plasticity variation Into consideration. That is, if salts are plastically varied, a contact between the salts is sufficiently ensured by a pressing process, to thereby prevent isolation by the fusion metal. Thus, if the plasticity variation of salts does not occur or the degree of the plasticity variation is not sufficient, the isolated salts are not removed in the eruption process of salts, which has bad influence on corrosion-resistance of porous metal.
Thus, considering the above points, it is preferable that a slat that can be used in the present invention is a one-value salt such as sodium chloride or potassium chloride having an excellent plasticity variation.
The size of the pores In the porous metal according o the present invention s decided by the size of the particle of salt powder and the distribution rate of the pores is decided by a mixture ratio of salt and metal powder. Thus, it is possible to properly adjust the size of the salt powder and a mixture ration of the salt and the metal powder according to a use of the porous metal. In general, it is preferable that the size of the particle of the salt powder is 0.05-5 mm and the size of the particle of the metal powder is 10-300 xcexcm. The metal powder is spherical, oval, needle-shaped or plate-shaped.
Raw powder of a mixture in which salt powder and metal powder are mixed at a predetermined ratio is filled into a mold at which the filled raw powder can be pressed at a predetermined pressure. Then, the mold is heated up to a temperature that is lower than the melting temperature of the salt powder and higher than that or the metal powder. In this case, it is preferable that an air transmission layer is formed on the lower portion of the mold so that air can be discharged through the bottom of the mold before the raw powder of the mixture is filled therein.
A plastic body that is fabricated by pressing and plastering silica powder as an example of the air transmission layer is set in the lower portion of the mold for pressing, and then the mixture raw powder is charged therein. Then, a pressing process is performed. The air transmission layer is a porous plastic body having a capability of inhaling air discharged from the pressed mixture raw powder, and should have no reaction with the fusion metal in the mixture raw powder.
If the mixture raw powder is pressed according to the pressing process in the case that there is no air transmission layer, the fusion metal is not minutely filled in between he salt particles, but pushed out through the cap of the mold by an internal air pressure in the mixture. It is preferable that silica powder of 1-10 xcexcm is pressingly plastered to use it as an air transmission layer.
Meanwhile, in the case that a thickness of the final product to be obtained is increased, an amount of the raw powder to be charged is increased. Accordingly, the size of the test sample is increased. As a resume, only the air transmission layer formed on the bottom of the mold is insufficient to discharge air. In this case, it is necessary to maintain the internal air pressure to be lower than the atmosphere, by removing the air transmission layer existing between the mixture raw powders in advance.
As a method of removing air existing in the mixture of the salt powder and the metal powder, for example, the mixture raw powder is filled in the mold, the mold is put in a vacuum hot press, and then a vacuum pump can be operated until the internal air pressure reaches a desired vacuum. Otherwise, the mixture raw powder as filled in a container with an air outlet, an inlet of the container is sealed, and then a vacuum pump can be operated until the internal air pressure reaches a desired vacuum.
Here, the vacuum of the mold filled with the mixture of the raw powders or that of the container can be adjusted according to the thickness or use of the porous metal to be obtained, which is preferably equal to or less than 200 mTorr.
At the state when the metal powder has been heated up to the temperature of the melting temperature or higher, the fusion metal such as aluminum continuously maintains the shape of the powder by an oxidation film of the aluminum metal surface.
At this state, the mold is pressed by the press. By the purpose of pressing the mold, an isolation of the salt particles is prevented in which salt is plastically deformed to make a contact between the salt particles sufficiently occur, and at the same time the fusion metal is minutely filled in between the salt particles, in which the oxidation film of the surface on the particle of the fusion metal is destructed. Also, if the pressing force is increased and thus the space between the salt particles is decreased, part of the fusion metal is pushed out through the air transmission layer. Accordingly, an improved pore ratio can be obtained as a result.
In the case that a pressure of a sufficient magnitude is not applied, the space between the salt particles is not filled minutely with the fusion metal. Accordingly, the fabricated porous metal nearly loses a mechanical strength. Thus, the magnitude of the pressing force should be 50 kg/cm2 or more when the temperature is 700xc2x0 C. By the experimental results, when the heating temperature is equal to or higher than 700xc2x0 C., it has appeared that a required magnitude of the pressing force is somewhat decreased.
Then, the mold is cooled, and the plastic body test piece separated from the mold is dipped into water to make salt erupt, to thereby obtain porous metal.
As described above, the porous metal making method according to the present invention appears to be similar to the prior art xe2x80x9cmethod for pressingly casting perform fusion metal of sodium chloridexe2x80x9d which is simply referred to as a press casting method, but has the following distinctive differences and merits when compared with the press casting method:
Firstly, in the case that the space between the salts constituting a perform is minute in the press casting method, the fusion metal cannot be penetrated even by the press. However, in the present invention, since metal powder is filled in between the salts, the fusion metal can be filled irrespective of the size of the space between the salts. As a result, the porous metal having minute pores can be fabricated, which is difficult to be obtained in the press casting method.
Secondly, the porous metal obtained by the press casting method has a big offset in a pore distribution according to the position of the metal due to the insufficient filling of the fusion metal. The porous metal obtained in the present invention has an extremely uniform pore distribution.
Finally, the press casting method can be applied only in the batch production system. However, in the present invention, a press is installed air. the middle of a sequential heating furnace to enable a sequential mass-production.